This invention relates to shaped medical devices comprising polymer hydrogels having improved mechanical properties. More particularly, the invention relates to shaped medical devices which can function as temporary implants which do not require additional surgical procedures for removal.
Shaped medical devices for insertion and/or implantation into the body have a variety of applications including drug delivery, tissue engineering, vascular surgery (e.g., angioplasty) and drainage (e.g., from the kidney). Such devices include vascular grafts, stents, catheters, cannulas, plugs, constrictors, tissue scaffolds, and tissue or biological encapsulants, and the like.
Typically, many of these devices are made of durable, non-degradable plastic materials such as polyurethanes, polyacrylates, silicone polymers, and the like. Typical stents, for example, are permanent devices which require a surgical procedure to remove and may cause discomfort. Shaped medical devices have also been disclosed which are made from biodegradable polymers. These remain stable in vivo for a period of time but eventually biodegrade into small fragments which are removed by the body by normal elimination in the urine or feces. In the case of ureteral stents, the fragmentation of such biodegradeable polymers into small pieces can obstruct the ureter and cause patient discomfort.
Typical biodegradable polymers used in these devices include polyesters, polyanhydrides and polyorthoesters which undergo hydrolytic chain cleavage, as disclosed in U.S. Pat. No. 5,085,629, crosslinked polysaccharide hydrogel polymers as disclosed in U.S. Pat. No. 5,057,606, and other ionically crosslinked hydrogels, as disclosed in U.S. Pat. Nos. 4,941,870, 4,286,341 and 4,878,907, the entirety of which are incorporated herein by reference. Crosslinked polysaccharide hydrogel polymers also are disclosed in EPA 0507604 A2.
EPA 0645150 A-1 describes hydrogel medical devices prepared from ionically crosslinked anionic polymers (e.g., polysaccharides such as calcium alginate) or ionically crosslinked cationic polymers (e.g., chitosan, cationic guar, cationic starch, and polyethylene amine). These devices can be rapidly disintegrated in vivo upon the administration of a chemical trigger material which displaces the crosslinking ions.
Hydrogels are water-swollen polymers which offer excellent biocompatibility and have a decreased tendency to induce thrombosis, encrustation, and inflammation. Unfortunately, the use of hydrogels in medical device applications has often been hindered by poor mechanical performance. Although many medical device applications for hydrogels exist where minimal stresses are encountered by the device in vivo, most applications require that the device survive high stresses during implantation. Inferior mechanical properties result from the swollen nature of hydrogels and the non-stress bearing nature of the swelling agent (e.g., aqueous fluids). Hydrogels suffer from low modulus, low yield stress, and low strength, when compared to non-swollen polymer systems.
In addition, ion sequestrants within certain body fluids, such as the urine, will degrade the hydrogel too quickly, preventing the use of such hydrogels in ureteral stent applications since a hydrogel stent will not hold its strength long enough to allow the device to function. Further, the amount of sequestrants in the urine is very difficult to control, even with the use of certain diets. As a result, the performance of a hydrogel stent in the ureter is variable from patient to patient.